The invention relates to a position measuring system and a measuring process for determining an absolute position.
In a large number of electric motors it is necessary to determine the absolute position of the rotor during startup so that when the rotor is turned on, the ampere-turns of its coils lie at a right angle to the axis of rotation so that the rotor starts up in the desired rotational direction with the desired torque. The range of the angle within which the absolute position must be known is dependent on the setup of the electric motor. In order to be able to install an angle measuring system flexibly, it is customary to determine the absolute position for commutation of an electric motor over one complete revolution. In subsequent operation the angle measuring system should output the position of the rotor with as high a precision as possible.
PCT Publication No. WO 89/11080 discloses a position measuring system with two spacing tracks whose spacing periods differ negligibly. The phase difference between the analog signals generated by scanning the two tracks is determined and from the detected phase differences, signals are generated that serve for commutation of an electric motor.
German Patent Publication No. DE 195 13 692 C1 discloses another position measuring system which outputs signals for commutation of a motor. The device includes a scale that has a first track that has only a single period per revolution of the scale, i.e., a low resolution track, and a second track that has many periods per revolution of the scale, i.e., a high resolution track. In a first operating mode the absolute position of the scale is determined by scanning the low resolution track and evaluating the analog signal generated from scanning the low resolution track. In a second operating mode, the scale is to rotated to a desired position. A switching unit switches the operation of the device from the first mode of operation to the second mode of operation. In the second mode of operation the high resolution track is scanned. In this second operating mode the desired position to which the scale is to be located is specified as a number of increments. An increment is defined in terms of one period or a fraction of one period. As an example, if one increment equals one period and if the desired position requires the scale to be rotated 100 increments in the second mode of operation, then the scale is revolved until such a time as 100 periods have been detected by a counter. If the switching unit has correctly placed the device in the second mode of operation, the scale will have been rotated only a fraction of one revolution because the high resolution track has many periods. However, if the switching unit has not correctly placed the device in the second mode of operation then the low resolution track is still being scanned and because it has only one period per revolution, the scale will be rotated 100 revolutions in order for the counter to count 100 increments. Thus if the switching unit does not properly operate, considerable mispositioning of the scale occurs.
Thus it is desirable to provide a position measuring system and a measuring process in which this type of grossly incorrect positioning is avoided. It is also desirable to provide a position measuring system and measuring process in which only slightly incorrect positioning is able to occur if a malfunction of the switching unit occurs.